There are a number of previous designs for microphones having directional beam patterns. Various microphones of a single transducer design are known to have omnidirectional beam patterns, while others are known to have unidirectional (cardioid) beam patterns, and while still others are known to have superdirectional (hypercardioid) beam patterns. These single transducer microphones are relatively inflexible in situations where sound sources are moving about and changing position in the pick-up pattern. The omnidirectional microphone generally picks up unwanted sound sources whereas the directional microphones may eliminate some sound sources that are desired to be heard.
To alleviate this problem microphones with electronically controllable beam direction were developed. Microphone arrays which are controlled as to beam direction are now conventional and generally include various numbers of unidirectional microphones which are phased and combined to provided an electronically controlled beam pattern. Such arrays can be electronically focused on a particular area to either include or exclude various sound sources.
As an example, U.S. Pat. No. 4,485,484 to Flanagan discloses a directional microphone system which arranges microphones so as to focus them on a prescribed volume in a large room such as an auditorium. As disclosed, the system by Flanagan is designed to only accept signals which emanate from the prescribed volume and to reject any signals which are received from outside the prescribed volume. The system utilizes two microphone arrays wherein the first array is placed along a first wall and the second array is placed along a second wall, or is placed on the first wall and spaced a predetermined distance away from the first array. A separate position locator is employed which determines the position of the speaker. The system is not ideal due to the phase interference that occurs between the transducer signals. If the microphone arrays are not an equal distance from the sound source location, the resulting signals are not uniform in sensitivity for all points within the desired focal volume.
Moreover, for those array systems which electronically align the phases of each element, the simultaneous control of the directivity of the pick-up pattern and gain of each element is problematical. During a program with moving sound sources where, for example, some sources are desired to be included and some sources are desired to be excluded, not only is the directivity but the gain of the system output important. But it has proven extremely difficult and expensive to produce an array of unidirectional transducers which can be phased correctly for significant directivity across the entire spectrum of gains which are desirable. Additionally, because of the complex relationship of each signal in an array with the others for phase alignment, a program manager cannot easily control the directivity and gain for such arrays simultaneously without noticeable discontinuities to an audience.